The present invention relates to carbon foam materials, and more particularly to such materials derived from petroleum pith starting materials.
U.S. patent application Ser. No. 09/453,729 filed Dec. 2, 1999 and entitled, xe2x80x9cCoal-Based Carbon Foamsxe2x80x9d, describes a family of high strength and abrasion resistant carbon foams having densities of preferably between about 0.1 g/cm3 and about 0.8 g/cm3 that are produced by the controlled heating of low cost, coal particulate, preferably up to xc2xc inch in diameter, in a xe2x80x9cmoldxe2x80x9d and under a non-oxidizing atmosphere. Because of the relatively low cost of the starting material, and the significantly high strength and other properties of these materials, such foams are highly economically desirable
So called, xe2x80x9ccarbonaceous mesophasexe2x80x9d materials are nematic forms of carbon commonly derived from isotropic petroleum or coal tar pitches using a variety of well known techniques. Such materials take the physical form of very fine particulate or powders and are commonly applied in the fabrication of high strength carbon fibers and the like.
Among the techniques used to prepare these materials are: 1) thermal polymerization patented by Singer, U.S. Pat. No. 4,005,183 and 2) a solvent extraction process using toluene, heptane, benzene or the like solvents to extract high molecular weight fractions from an isotropic pitch as described by Riggs and Diefendorf in U.S. Pat. No. 4,208,267.
According to the Singer technique, an isotropic petroleum pitch is heated to a temperature of between 400 and 450xc2x0 C. under an inert atmosphere for about 40 hours. Under this treatment, about 50% of the isotropic phase is converted to an anisotropic phase. Because of the greater density of the latter material, it is collected at the bottom of the heating vessel. The product of this process exhibits a broad molecular weight and a high (240+xc2x0 C.) and ill defined softening point.
When treated in accordance with the Riggs and Diefendorf technique the solvent extract can be converted to a mesophase material by heating for only about 10 minutes at a temperature of between about 230 and 400xc2x0 C.
Both of these processes begin with a feedstock comprising an isotropic pitch that is a by-product of the petroleum and coal tar industries. Such feedstocks conventionally exhibit softening points in the range of from about 240xc2x0 C. to about 280xc2x0 C. While such feedstocks are relatively inexpensive, they often and generally do contain insoluble carbon solids that decrease the mechanical properties of carbon fibers manufactured therefrom, and are therefore considered highly inferior for this purpose.
Subsequent developments included the design of mesophase production approaches involving the treatment of high purity starting materials such as pure aromatic hydrocarbons, for example, naphthalene that have been developed by Mitsubishi Gas and Chemical of Japan. While more suited to the preparation of carbon fibers, these materials are considerably more expensive than those derived from petroleum or coal tar pitch and therefore not economically suitable for many applications, such as the production of carbon foam, due to cost.
While the coal-based starting materials of U.S. patent application Ser. No. 09/453,729 provide a ready source of low cost starting material for the production of highly desirable carbon foams, it has been considered that other similar low cost materials such as petroleum pitch or coal tar pitch might also be used as starting materials for the production of competitive carbon foams. The application of processes similar to those used to obtain coal-based carbon foams from coal particulate to the production of low softening point pitch-based mesophase carbon materials has proven problematic.
Apparently because of the relatively low and broad softening points of isotropic coal tar and petroleum pitch materials (generally in the range of from about 240xc2x0 C. to about 280xc2x0 C.), the application of foaming processes such as those described in aforementioned U.S. patent application Ser. No. 09/453,729 is very difficult and attempts to foam them in this fashion have generally failed. Such failure is primarily due to the fact that, although the mesophase materials derived from such xe2x80x9clow softening pointxe2x80x9d petroleum and coal tar pitches, as just described, can be xe2x80x9cfoamedxe2x80x9d using processes similar to those described in connection with the production of coal-based carbon foams, they tend to xe2x80x9cslumpxe2x80x9d or collapse upon themselves during the foaming process resulting in a relatively dense, if somewhat porous, mass, unless extremely tight process controls are applied. Even when such care is taken, the variability of such petroleum and coal tar pitches often results in failure of the foaming operation. Thus such foams have not until now been found highly useful as low density and high strength structural, insulating, ablation resistant or abrasive foam materials.
It is therefore an object of the present invention to provide a useful and reliable method for the production of carbon foams from isotropic petroleum and coal tar pitch starting materials.
It is another object of the present invention to provide a method for the production of carbon foam from isotropic petroleum and coal tar pitch that is relatively easy to implement and control and therefore economically and commercially feasible.
It is yet another object of the present invention to produce a low density, high strength carbon foam from isotropic petroleum and coal tar pitch-based starting materials.
According to the present invention there is provided a method for the production of carbon foam using as a starting material an isotropic coal tar or petroleum pitch having a softening point above about 300xc2x0 C. Additionally, according to the present invention there are provided petroleum or coal tar pitch-based cellular or porous products, i.e. foams, having a density of preferably between about 0.1 g/cm3 and about 0.8 g/cm3 that are produced by the controlled heating of mesophase carbon materials derived from coal tar or petroleum pitches that exhibit softening points in excess of about 300xc2x0 C., and preferably between about 300 and about 400xc2x0 C., in a xe2x80x9cmoldxe2x80x9d and under a non-oxidizing atmosphere. The porous product thereby produced, preferably as a net shape or near net shape, can be machined, adhered and otherwise fabricated to produce a wide variety of low cost, low density products, or used in its preformed shape as a filter, heat or electrical insulator etc. Such cellular products, without further treatment and/or the addition of strengthening additives exhibit compressive strengths of up to and above about 4000 psi, and even up to 6000 psi. Impregnation with appropriate materials or the incorporation of various strength improving additives can further increase the compressive, tensile and other properties of these cellular materials. Further treatment by carbonization or graphitization yields cellular products that can be used as electrical or heat conductors.